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Oil and Gas Reserves Information (Unaudited) - Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Extractive Industries [Abstract]      
Standardized Measure, Beginning of Year $ 372,139 $ 1,090,660 $ 807,217
Net change in sales price, net of production costs (45,379) (751,774) 5,911
Development costs incurred during the year which were previously estimated 45,648 157,390 344,590
Revisions of quantity estimates 113,583 (111,454) (40,993)
Accretion of discount 37,251 114,427 105,400
Changes in future development and abandonment costs 5,315 14,901 (10,909)
Changes in timing and other (38,071) (44,439) (19,028)
Extensions and discoveries 70,149 56,216 163,559
Sales of minerals in place (22,449) (43,694) 0
Sales, net of production costs (107,253) (163,336) (458,254)
Net changes in income taxes (1,658) 53,242 193,167
Standardized Measure, End of Year $ 429,275 $ 372,139 $ 1,090,660